A popular type of refrigerant compressor for use in vehicle air conditioning systems involves a wobble or nutating drive mechanism to provide infinitely variable displacement. In this type of compressor, a plurality of cylinders are equally angularly spaced about a cylinder block and compressor housing, and equally radially spaced from the axis of a central drive hub. A piston is mounted for reciprocating motion in each of the cylinders. A piston rod connects each piston to a non-rotatable socket or wobble plate that provides the nutating motion in response to a rotating drive shaft attached to the central drive hub. The driving of the socket plate in a nutating path serves to impart the linear reciprocating motion to the pistons, thereby providing proper compressor operation. By varying the angle of the socket plate relative to the drive hub, in response to internal differential gas pressure, the stroke of the pistons and, therefore, the displacement or capacity of the compressor is varied.
A drive journal in the form of a disc and associated with the central drive hub is the rotating component that actually engages the socket plate. The journal is in substantially parallel driving relationship with the socket plate at all times, and thus the angle of both the socket plate and journal relative to the drive hub is substantially the same. Therefore, the stroke of the pistons, and accordingly the displacement or capacity of the compressor, is in effect a function of the angle of the journal.
Thus, as the journal rotates about its axis, the socket plate is held against rotation and is forced by the journal so as to travel in the nutating path. When the axes of the socket plate and journal are substantially coincident with the axis of the central drive hub, the compressor is operating at zero compression or zero stroke. Where the axes of the socket plate and journal are at their extreme allowable angle relative to the axis of the drive hub, the compressor is operating at full compression or full stroke. It can be appreciated that the stroke of the pistons is infinitely adjustable between zero stroke and full stroke as the socket plate and journal pivot from axial coincidence with the central drive hub to the extreme position.
As mentioned above, the angle of the socket plate/journal assembly is basically a function of a difference in internal refrigerant gas pressures. More particularly, the differential between the internal housing or crankcase pressure and the suction pressure of the compressor determines the angle of the assembly, and accordingly the stroke or displacement of the compressor. While prior art compressors thus rely primarily on the net force created by the compressor housing-suction pressure differential to pivot the assembly, a split ring return spring is also provided simply to initiate a short-stroke movement from the zero stroke position towards the full stroke position.
In order for the socket plate/journal assembly to operate properly, the journal must pivot during operation about an axis defined by a pivot pin spaced from the central axis of the compressor, and extending substantially perpendicular to the direction of piston movement. During pivoting, the pivot pin moves along its supporting kidney slot in order to assist in maintaining alignment of the pistons within the mating cylinders. There is also provided in present socket plate compressors, a pivot pin/sleeve arrangement for guiding the journal's translational movement along the rotating central drive shaft. For a more complete review of this arrangement, reference is made to FIGS. 1 and 2 in U.S. Pat. No. 4,815,358 to Smith, issued Mar. 28, 1989, and owned by the present assignee.
While this prior art journal mounting arrangement is thus successful in directing the pivoting action of the journal, as determined by the differential gas pressure, it provides minimal true upstroke and destroke control. In this regard, it is now recognized that the relationship between differential refrigerant gas pressure and piston stroke is non-linear, and thus difficult to accurately control along the full range of movement. As referenced above, the split ring return spring in the prior art compressors tries to alleviate the problem but doesn't fully solve it, since its spring action is effective only for a very short distance on the upstroke, that is, at the start of the movement from the zero stroke position. Once this return spring becomes disengaged from the sleeve just after initiation of travel along the drive shaft towards the full stroke position, the ability to assist in providing control is, of course, lost.
Accordingly, there is a need for an improved assembly to provide stroke control along the full range of movement of the journal, as opposed to initiation and thus control, for only the very short distance at the start of the upstroke. If this can be accomplished, it would allow the compressor to upstroke and destroke more efficiently. Ideally, the improved control assembly would provide the proper complementary stroking force to coact with the housing-suction pressure differential to provide more efficient compressor operation. Such an assembly would thus improve stroke controllability by linearizing the relationship between the overall upstroking/destroking control force and the desired corresponding piston stroke. Furthermore, it would be desirable to integrate the stroke control assembly with the compressor torque restraint mechanism, such as disclosed and claimed in the copending Ebbing et al U.S. application entitled RZEPPA JOINT SOCKET PLATE TORQUE RESTRAINT ASSEMBLY FOR A VARIABLE DISPLACEMENT COMPRESSOR, filed Apr. 5, 1990, Ser. No. 07/504,817.